1. Field of the Invention
The present invention generally relates to a non-linear load driving circuit and a controller, and more particularly, to a non-linear load driving circuit capable of reducing overshoot and a controller.
2. Description of Related Art
FIG. 1 is a diagram of a conventional light emitting diode (LED) driving circuit. Referring to FIG. 1, the LED driving circuit includes a controller 10, a conversion circuit 50, and an LED module 60. The conversion circuit 50 is coupled to an input voltage source Vin. The controller 10 generates a control signal Sc to control the conversion circuit 50 to transmit a power from the input voltage source Vin to an output terminal. The output terminal of the conversion circuit 50 is coupled to the LED module 60 to apply an output voltage Vout on the LED module 60, so as to make an output current Iout to flow through the LED module 60 and make the LED module 60 thus to emit light. The output current Iout also flows through a current detection resistor 65 to generate a current feedback signal IFB.
The controller 10 includes an error amplifier 12, a triangular wave generator 14, a pulse width modulation (PWM) comparator 16, and a driving circuit 18. The error amplifier 12 receives the current feedback signal IFB and a reference voltage Vr and generates an error amplified signal Vea according to the current feedback signal IFB and the reference voltage Vr. The triangular wave generator 14 generates a triangular wave signal for the PWM comparator 16. The PWM comparator 16 also receives the error amplified signal Vea and generates a PWM signal for the driving circuit 18 according to the comparison of the error amplified signal Vea and the triangular wave signal. The driving circuit 18 generates the control signal Sc according to the PWM signal generated by the PWM comparator 16.
Generally speaking, the controller 10 stabilizes the output current Iout at a predetermined output current Io. In this case, the output voltage Vout is also stabilized at a predetermined output voltage Vo. However, the error amplifier 12 compares the current feedback signal IFB with the reference voltage Vr and integrates the error between foregoing two signals to adjust the level of the error amplified signal Vea. Such a feedback control process causes the output current Tout and the output voltage Vout to oscillate around and gradually converge towards the predetermined output current Io and the predetermined output voltage Vo (i.e., the amplitudes thereof decrease).
FIG. 2 illustrates waveforms of signals in the LED driving circuit in FIG. 1 during a startup process of the LED driving circuit. Before the controller 10 is started, the output voltage Vout, the output current Tout, the error amplified signal Vea, and the control signal Sc are all at low levels. When the controller 10 is started at the time point T0, since the output current Tout is much lower than the predetermined output current Io, the error amplified signal Vea increases quickly, and accordingly the duty cycle of the control signal Sc also increases quickly. Herein the output voltage Vout also starts to increase. Before the output voltage Vout reaches the threshold voltage Vf of the LED module 60 (i.e., before the time point T1), the output current Tout flowing through the LED module 60 remains at the zero level. Because the output current Tout remains much lower than the predetermined output current Io during the period T0-T1, the error amplified signal Vea increases to its highest level. The output current Iout starts to increase at time point T1 and reaches the predetermined output current Io at time point T2.
At the time point T2, the output current Tout is higher than the predetermined output current Io, so that the error amplifier 12 starts to pull down the level of the error amplified signal Vea. However, due to the integration, the error amplified signal Vea cannot drop to an error steady value Veao (i.e., the level of the error amplified signal Vea corresponding to the output current Iout when the output current Tout is stabilized at the predetermined output current Io) directly. As a result, the duty cycle of the control signal Sc is too large, so that the output current Tout continues to increase until the error amplified signal Vea is lower than the error steady value Veao and then the duty cycle of the control signal Sc is too small. At the time point T3, the output current Tout is lower than the predetermined output current Io again. Then, the error amplified signal Vea increases again and exceeds the error steady value Veao. Foregoing process continues until time point T4, at which the output current Tout, the output voltage Vout, and the error amplified signal Vea respectively converge to the corresponding predetermined output current Io, predetermined output voltage Vo, and error steady value Veao.
Besides during the startup process of the controller 10, overshoot of output current is also produced when the LED module performs burst dimming. Moreover, when the output current Iout reaches the predetermined output current Io for the first time, the error amplified signal Vea remains at the highest level, so that very large overshoots are produced in the output current Iout and output voltage Vout. The overlarge current and voltage overshoots reduce the stability of the circuit and may even damage the circuit.